In a known fingerprint detection apparatus, the finger under investigation is usually pressed against a flat surface, such as a side of a prism or a glass plate, and the ridge and valley pattern of the finger tip is illuminated with an interrogating light beam. In the optical devices which employ prisms, the prism has a first surface upon which a finger is placed, a second fingerprint viewing surface disposed at an acute angle to the first surface, and a third illumination surface through which light is directed into the prism. The incident beam of light is reflected from the first surface and exits through the fingerprint viewing-surface. An image producing lens or lens system is provided for receiving the beam reflected from the valleys of the subject fingerprint and for producing an image of the subject fingerprint at an image sensor, e.g. a charge coupled device (CCD) or the like for coverting the fingerprint image into an electric signal. An amplifying/analysing signal processing circuit and a monitor for displaying the fingerprint image are also provided. In some cases, the illumination surface is at an acute angle to the first surface, as seen for example, in U.S. Pat. Nos. 5,187,482 and 5,187,748. In other cases, the illumination surface is parallel to the first surface, as seen for example, in U.S. Pat. Nos. 4,924,085, 5,109,427 and 5,233,404.
Fingerprint identification devices of this nature are generally used to control building-access or information-access of individuals to buildings, rooms, and devices such as computer terminals.
One of the problems associated with fingerprint sensors concerns the reliable and accurate transformation of ridge and valley pattern of the fingertip into electrical or optical signals to be stored in a digital format. Optical systems as described above, for example using a prism, require sophisticated equipment and tend to be bulky and costly.
Using an optical contact imaging device, the height from platen to imaging circuit is a function of a trapezoid formed by the CCD at one base thereof and the lens at the other base thereof, assuming lenses of comparable focal power. A large base gives rise to a large trapezoid compared to the two trapezoids defined by two smaller bases and so forth. By increasing the number of imaging paths, the distance from platen to imaging circuit is reduced.